The compressible rubber blanket for offset printing is used to receive ink from a plate and transfer it to an object to be printed such as paper. FIG. 4 is a cross-sectional view showing the structure of the main parts of the conventional compressible rubber blanket for offset printing. In the figure, 1 is a surface rubber layer which received and transfers ink. Surface rubber layer 1 is laminated on reinforcing layer 2 sequentially through compressible layer 3 and woven cloth 4. In reinforcing layer 2, three woven cloths 2a are integrally laminated with solid rubber layers 2b which are interposed between each woven cloth 2a. Compressible layer 3 is to absorb shock such as mechanical impact and to improve the reproducibility of a plate. Compressible layer 3 is formed of porous rubber for instance. Woven cloths 2a and 4 are formed of cotton cloth or synthetic fiber.
The compressible rubber blanket for offset printing constructed in this way is mounted on a metallic blanket cylinder when it is used, with a constant tension being applied thereto. Accordingly, a process for preventing the elongation due to the tension is applied to maintain a predetermined strength. To fully exhibit the action for preventing the elongation due to the tension, usually a tensile strength of 65 Kgf/cm is required.
On the other hand, the thickness of one blanket is typically about 1.9 mm. In addition, the total number of woven cloths 2a and 4 for reinforcement is often 4 or larger.
In FIG. 4, four woven cloths 2a and 4 are used for the traditional compressible rubber blanket for offset printing, and the thickness of the whole blanket is about 1.9 mm. Compressible layer 3 is a base for designing a compressible rubber blanket for offset printing, and in this case that having a thickness of 0.25 to 0.4 mm is considered to be the best. The material and thickness of compressible layer 3 is determined in consideration of the reproducibility of images of the plate, the running characteristics of forms in the offset rotary press, and absorption of shock such as mechanical impact.
It is said that surface rubber layer 1 performing the transfer of ink requires a thickness of 0.25 mm or greater at the minimum. If surface rubber layer 1 is too thin, the texture of the uppermost woven cloth 4 appears on the printed matter, causing printing failure. In order to prevent this phenomenon, usually a comparatively thin woven cloth 4 having a thickness of about 0.2 mm is used. As the lower woven cloths 2a, rather thick ones of the order of 0.3 to 0.4 mm are used to fully exhibit strength. For such limitations on surface layer, etc., the thickness of solid rubber layer 2b of the conventional compressible rubber blanket for offset printing naturally becomes thin, usually in the order of 0.05 mm. And, solid rubber layer 2b is dedicated to the role of bonding woven cloths 2a with each other.
Incidentally, the thickness of each layer of the typical conventional compressible rubber blanket for offset printing is as follows for instance.
______________________________________ Surface layer 1 0.25 mm Woven cloth 4 0.21 mm Compressible layer 3 0.25 mm Woven cloth 2a 0.4 mm Solid rubber layer 2b 0.05 mm Woven cloth 2a 0.3 mm Solid rubber layer 2b 0.05 mm Woven cloth 2a 0.4 mm Total thickness 1.91 mm ______________________________________
For this compressible rubber blanket for offset printing, the total thickness of the solid rubber layer is 0.1 mm, and it has the following drawbacks.
1) Since solid rubber layer 2b is thin, the action of absorbing a shock such as a mechanical impact is insufficient, and particularly in old printing machines or high-speed printing machines, stripes called shock marks appear on the printed matter.
2) Woven cloth 2a such as cotton cloth is crushed when applied with a pressure and causes a permanent deformation, whereby the thickness of the blanket is reduced during printing (this is called sinking). As a result, the pressure during printing decreases and the transfer of ink also decreases, and thus a required ink density cannot be obtained on the printed matter. Such condition occurs, the printing machine should be stopped and a packing should be added under the blanket to compensate the reduced thickness. For this, the operating efficiency of the printing machine is reduced.
3) When a paper break trouble occurs during printing, folded papers are pressed against the blanket surface, which locally receives a large deformation. Whereupon, this portion causes a permanent distortion and becomes uneven, and thus printing is unabled.
In the usual printing, it is compressible layer having a large elastic recovery ability which is applied with the printing pressure and deforms, but at the time of smashing (instantaneous large deformation), it cannot be treated only by the deformation of the compressible layer and the strong pressure reaches even to the woven cloth, resulting in the crush of the woven cloth to cause a permanent distortion. As a result, printing is unabled.
4) If a printing material of a wide paper width is printed after a large number of printing materials of a narrow paper width have been printed, a sinking occurs in the portion corresponding to that of the narrow paper wide, and the ink density becomes low. Particularly, a linear shading occurs in the end portion of the paper width, causing print failure.
As a result of our various studies, it was found that woven cloths 2a and 4 cause a shock such as a mechanical impact, sinking and the occurrence of impressions due to paper break or the like, which result in such defects. It was also found that, in the conventional compressible rubber blanket for offset printing shown in FIG. 4, a shock such as a mechanical impact cannot fully be absorbed since solid rubber layers 2b between woven cloths 2a are thin.
In addition, the lowermost one among the woven cloths 2a is experiences particularly large permanent deformation because it is used in directly contact with the metallic cylinder. In contrast to this, woven cloth 2a nearer to compressible layer 3 has less degree of the permanent deformation.